Process for thru-hole etching of a metal plastic



R. M. WHITE 3,502,519

PROCESS FOR THRU-HOLE' momma OF A METAL PLASTIC March 24, 1970 Filed De c. 8. 1966 ROBERT M. WHITE INVENTOR. 9% 69 BY @444.

A TTOR/VEYS United States Patent 3,502,519 PROCESS FOR THRU-HOLE ETCHING OF A METAL PLASTIC Robert M. White, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Dec. 8, 1966, Ser. No. 600,226 Int. Cl. C2311 3/04 US. Cl. 156-5 3 Claims ABSTRACT OF THE DISCLOSURE A method for improving the lateral cross-sectional profile of an etched-through article in which the article is submerged in an etchant bath with its upper resist coated surface slightly below the surface of the bath and an external etchant spray is impinged thereon to agitate the bath.

Printed circuit boards, light modulating reticles, ribbon heater elements, and miniature sheet metal parts are typical examples of products best manufactured by chemical milling techniques; these particular products are also members of the generic class of chemically milled objects called etched-through articles. Members of this class are readily distinguishable from other chemically milled objects, such as, for example, relief plates, simply because the unmasked portions of the workpiece from which the etchedthrough articles are manufactured, are completely destroyed during the etching process. That is to say, the caustic action of the etchant is permitted to continue until breakthrough occurs, thus leaving behind an etched-through article similar in shape to that of the mask.

To date, difiiculty has been experienced in the economical manufacture of relatively thick etched-through parts (i.e. parts having a thickness greater than 5 mils) when a substantially perpendicular edge profile is specified. The major contributing problem is produced by, what is commonly known as, lateral etching. During the etching process the etchant first attacks the vulnerable portions of the workpiece in a direction normal to its surface. After a moderate depth has been attained, thus exposing new and unprotected material to the etchant, etching in a lateral direction commences. The degree to which lateral etching occurs is, of course, a function of the material being etched, the specific etchant, and the manner in which the etchant is applied (i.e. spray or bath). The combination of normal and lateral etching quite naturally produces an angular edge profile coming to a sharp edge at the point at which breakthrough occurs. In cases where etching is performed simultaneously from both sides of the workpiece it is evident that the edge profile will be chevron shaped.

The prior art methods of producing chemically milled parts with a substantially perpendicular lateral profile is to either subject the workpiece to a sophisticated and complex etching procedure, such as disclosed by Langsford in US. 2,802,726 or to subject the etched-through article to a manual finishing procedure which simply consists of abrading the sharp edges into conformity. Both methods add considerably to the manufacturing cost.

Now in accordance with the present invention there is provided a relatively simple and economical process for improving (i.e. making perpendicular) the lateral profile of etched-through parts. Basically the process consists of the simultaneous subjection of etched-through parts to an etchant spray and etchant bath. Quite unexpectedly, the process results in a dissolution of the sharp edges at a much higher rate than the smooth portions thereby producing the desired perpendicular lateral profile.

One object of this invention is to provide an economical process for improving the lateral profile of an etchedthrough article.

Another object of this invention is to provide an improved process for simultaneously finishing the edges of a plurality of discrete chemically milled parts.

Another object of this invention is to provide a spray/ bath etching process for straightening the profile of etchedthrough parts.

Another object of this invention is to provide an improved means of agitating an etchant bath to promote etching efficiency.

Still another object of this invention lies in providing a novel etching procedure which may be readily incorporated with conventional techniques for finishing the edges of etched-through parts.

Yet another object of this invention is to provide an improved chemical milling process suitable for producing articles with substantially perpendicular edges.

Other objects of the present invention will be readily apparent to those skilled in the art from the following description and appended drawings, wherein is illustrated an especially valuable embodiment of the invention in which:

FIGURE 1 is a diagrammatic view of a horizontal etching machine illustrating the first stage of operation.

FIGURE 2 is a similar view of the same apparatus and members in a subsequent stage of operation.

FIGURE 3 is a similar view of the same apparatus modified to carry out one step of the process of the present invention.

FIGURE 4 is a lateral cross-sectional profile of an etched-through article prior to treatment in accordance with the present invention.

FIGURE 5 is a lateral cross-sectional profile of an etched-through article subsequent to treatment in accordance with the present invention.

General apparatus for implementing and carrying out the process of the present invention are shown in FIG- URES l, 2 and 3. As shown in these figures a workpiece, tag. of metal, generally designated 10, having a plurality of registered patterns of resist material 11 on the top and bottom surfaces thereof, is longitudinally and horizontally transported by rollers 13 through a horizontal spray etching machine comprising a plurality of spray etching sources 12 arranged and designed to deliver conical sprays upon the surface to be etched. The etchant can be any one of the many commercially available etchants suitable for use with the material of the workpiece to be etched. The nozzles are spaced at regular intervals throughout the machine such that the etchant is uniformly supplied to all surface portions of the workpiece. Means (not shown) for controlling the drive of rollers 13, and thus, the advancement of the workpiece is also provided. Etching of the vulerable portions of the surface, i.e. those without resist thereon, is permitted to continue until such a depth is attained that breakthrough is near. At this point the workpiece 10 is removed from the etchant environment, placed on a non-soluble screen 14, having a mesh of such size that the etched-through articles when formed do not fall through the screen, and the workpiece is subjected to the etchant environment for a second time as shown in FIG- URE 2. Etching proceeds until breakthrough occurs thus rendering a plurality of etched-through articles 16, each having chevron shaped edges, such as illustrated in FIG- URE 4. The screen 14, supporting articles 16, is then placed in a non-soluble tray 17, having a depth substantially equal to the combined thickness of the screen 14 and etched-through articles 16 supported thereby, and the articles are subjected for a third time to the etchant spray environment of the etching machine. This time, however,

the use of lower spray sources is unnecessary. In each of these etchant environments it will be noted that the workpiece or etched-through article, as the case may be, is moved along a determined path at a controlled rate of speed so that the angle of impingement of the etchant sprays changes relative to the resist coated surface of the workpiece and an effective and uniform etch is obtained. During the spraying of etchant emitted from the upper spray sources, the tray 17 is rapidly filled with etchant, thus submerging the articles 16 contained therein. The spraying of the submerged etched-through articles is continued until the desired lateral profile is attained.

It is most common that the workpiece 10 is in the form of a plate having parallel flat opposed surfaces which are coated with the resist material. It should be understood, however, that workpieces of different configurations can be utilized in the practice of the present invention, for example, workpieces having opposed curved resist coated surfaces i.e. one convex and one concave, or two convex, or two concave are also successfully etched with the disclosed method. In each case, one of the resist coated surfaces of the submerged etched-through article is supported slightly below the surface of the etchant bath.

The instant invention involves the discovery of a novel etching technique. It has been found that when an etchedthrough metal article having resist material on two opposed surfaces only, i.e. the top and bottom surfaces, and having a chevron shaped lateral profile, is fully immersed in a shallow bath of etchant with the uppermost resist bearing surface parallel to and slightly below the surface of the bath, that upon directing an external etchant spray upon the uppermost surface of the bath the agitated etchant reacts upon the unprotected sides of the etched-through article in an unexpected manner; i.e., the combination im- Inersion/ spray etching process tends to etch away the sharp edge of the chevron at a much higher rate than it does the base near the resist coatings, thus improving both the rectilinearity of the sides and the perpendicularity of the sides with respect to the resist bearing surfaces.

Referring to FIGURE 4, the lateral profile of a typical etched-through article is shown having chevron shaped edges. The degree of perpendicularity, herein referred to as the perpendicularity ratio, is the ratio of the thickness of the article to the distance between the true perpendicular and the outermost edge, or simply B/A. Shown in FIG- URE is the lateral profile of an etched-through article subsequent to being acted upon in accordance with the instant invention.

EXAMPLE A pair of identical and registered resist patterns were formed on the top and bottom surfaces of a T. 316 stainless steel plate having a thickness of 30 mils. The plate was then spray etched by conventional techniques, using 42 B. FeCl at 130 F. as an etchant, until breakthrough occurred. At this point the lateral profile had a perpendicularity ratio of 4:1. The etched-through article was then subjected to a spray/immersion process in accordance with the instant invention. The improvement in the perpendicularity ratio was found to be approximately 500% or a ratio of 20: 1.

From the foregoing description it-will be readily apparent to those skilled in the art that the process of the present inventionembodies many advantages for industry. For example: (1) the process is readily adapted to fully automatic operation; (2) essentially no new apparatus is required; (3) the process requires substantially less time to produce the desired profile than previously required, and

(4) several differently shaped etched-through articles may be processed simultaneously.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A process for improving the lateral profile of an etched-through metal article having upper and lower substantially parallel surfaces coated with an etchant resist material, said process comprising immersing said article in an etchant bath such that the upper coated surface of said article is substantially parallel and slightly below the surface of said bath, and while so immersed, directing an external spray of etchant onto the etchant bath surface to agitate the etchant.

2. The process of forming an etched-through metal article having a substantially perpendicular lateral profile from a workpiece having at least two substantially parallel etchant resist coated surfaces with identical registered masked and unmasked areas thereon, said process comprising:

continually moving said workpiece along a determined path at a controlled speed,

directing an etchant in the form of fluid pressure induced sprays, which are fixed in position relative to said path, to impinge against said surfaces at angles which vary in accordance with the position of the moving workpiece relative to the sprays, until the unmasked areas have been etched to such a depth that breakthrough occurs, thus forming at least one etched-through article,

movably supporting said one article with its resist coated surfaces positioned horizontally in a tray having an effective depth in excess of the distance between said surfaces of said one article,

filling the tray with an etchant to form a bath of a depth sufficient to submerge slightly the upper resist coated surface of said one article, moving said one article within the etchant bath along a determined path at a controlled speed, and

directing an etchant in the form of fluid pressure induced sprays to impinge on the upper surface of the etchant bath to agitate the etchant in a manner which varies relative to said one article in accordance with the position of said one article relative to the sprays, until the lateral profile of said one article has attained the desired perpendicularity ratio.

3. The invention of claim 2 and wherein said one article is positioned in the tray prior to directing the etchant spray thereon whereby etchant fills the tray to said sufiicient depth and thereafter agitates the bath in the region of etched-through surfaces.

References Cited UNITED STATES PATENTS 2,828,192 3/1958 Langsfeld '156-5 OTHER REFERENCES Machinery, February 1966, pp. 92, Chemical Blanking.

JACOB H. STEINBERG, Primary Examiner US Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,502,519 Dated March 24. 1970 Inventor(s) Robert White It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

q In the Specification In the patent at the heading of the specification and drawing the title "PROCESS FOR THRU-HOLE ETCHING OF A METAL PLASTIC should be CHEMICAL MILLING PROCESS as indicated at the top of the first page of the application.

In the patent the word "vulerable'f should be "vulnerable" as indicated on page 4 line 15 of the application.

SIGNED 5m SEALED AUG 251970 SEAL) Amt:

mwf mm; x. .12. 0M Oonmissioner of Patents 

